Answer: This is because of the decreasing levels of Testosterone in Bill.
Explanation: Testosterone is the primary hormone in male humans and is often called a sex hormone. It is a anabolic steroid and directly leads to the development of a male sexual characteristics including higher muscle mass and production of semen.
As such, testosterone has a direct impact on male libido and even mood swings.
In the case of Bill, as he has gotten older, his testosterone levels have decreased, which makes him less angry than before.
